1. Field of the Invention
The present invention relates to a sheet feeding apparatus, and to an image forming apparatus having the sheet feeding apparatus. More particularly, the present invention relates to a structure adapted to blow air against an end surface of a sheet stack to separate and supply sheets of the sheet stack one by one.
2. Description of the Related Art
A conventional image forming apparatus, such as a copying machine or a printer, has a sheet feeding apparatus adapted to separate sheets stacked on a sheet stacking unit from an uppermost sheet one by one and to feed the separated sheets to an image forming unit one by one.
As discussed in, for example, U.S. Pat. No. 5,645,274, a sheet feeding apparatus blows gas (mainly air) against a sheet stack on a sheet stacking unit to float and separate a plurality of sheets. Then, the sheet feeding apparatus suctions an uppermost one of the sheets to a conveyance belt to feed the suctioned sheet.
FIG. 5 illustrates such a sheet feeding apparatus adapted to separate a sheet by using air and to feed the separated sheet by suctioning the separated sheet to the conveyance belt.
A repository 11 shown in FIG. 5 stores a plurality of sheets S and is provided in an image forming apparatus body (not shown) to be drawable therefrom. A sheet tray 12, on which the sheets S are mounted, is provided in the repository 11 to be movable up and down. Also, a rear end regulating plate 13 and side end regulating plates 14 are provided in the repository 11. The rear end regulating plate 13 regulates a rear end position of a sheet stack SA, which is an upstream end position in a sheet feeding direction. The side end regulating plates 14 regulate side end positions of the sheet stack SA, which are end positions in a width direction perpendicular to the sheet feeding direction. Additionally, slide rails 15 used to draw the repository 11 from the image forming apparatus body (not shown) are provided in the repository 11. The rear end regulating plate 13 and each of the side end regulating plates 14 are movable in the sheet feeding direction and in the width direction perpendicular to the sheet feeding direction, respectively, according to the size of sheets mounted on the sheet tray 12.
A suction conveyance belt 21 conveys a sheet by suctioning the sheet thereto. A suction fan 36 is adapted to suction a sheet to the suction conveyance belt 21. An air blowing unit 30 blows air to a front end surface of the sheet stack SA at a downstream side in the sheet feeding direction. The air blowing unit 30 includes a separation fan 31, a separation duct 32, a blowing nozzle 33, and a separation nozzle 34.
When a user puts the repository 11 into the image forming apparatus body (not shown) after the user draws the repository 11 out of the image forming apparatus body and sets the sheets S in the repository 11 in the sheet feeding apparatus, the sheet tray 12 is lifted by a drive unit (not shown) in the direction of an arrow A, as shown in FIG. 6. Then, the sheet tray 12 stops at a position at which a distance B from the top surface of the sheet stack SA to the suction conveyance belt 21 has a predetermined value. Subsequently, the sheet feeding apparatus waits for a feed signal.
Next, when a feed signal is input to the sheet feeding apparatus, the separation fan 31 operates to suction air in the direction of an arrow C shown in FIG. 7. This air is blown in the directions of arrows D and E from the blowing nozzle 33 and the separation nozzle 34, respectively, through the separation duct 32 to the front end surface of the sheet stack SA. Consequently, several sheets Sa of the sheet stack SA are floated. On the other hand, the suction fan 36 operates to blow air in the direction of an arrow F shown in FIG. 7. At that time, a suction shutter 37 provided in the suction duct 60 having a suction opening is closed.
Subsequently, after the lapse of a predetermined time since the feed signal is input to the sheet feeding apparatus, when a floating state of the several sheets Sa is stabilized, the suction shutter 37 is rotated in the direction of an arrow G, as shown in FIG. 8. Consequently, a suctioning force acting in the direction of an arrow H from suction holes (not shown) formed in the suction conveyance belt 21 is generated by the suction fan 36. The uppermost sheet Sb of the floated sheets Sa is suctioned to the suction conveyance belt 21.
Finally, the sheet Sb is fed in the direction of an arrow K together with the suction conveyance belt 21 by a belt drive roller 41 rotating in the direction of an arrow J shown in FIG. 9. Thereafter, the sheet Sb is transported to the subsequent conveyance path by a drawing roller pair 42, the rollers of which rotate in the directions of arrows L and M, respectively.
Meanwhile, in a case where such a conventional sheet feeding apparatus starts feeding the sheet Sb before the sheet Sb is completely suctioned to the suction conveyance belt 21, the following problems may occur. That is, conveyance failures, such as a jam at which a sheet is jammed while being fed, a skew at which a sheet is conveyed in a slanting direction, and a positional deviation at which the position of a sheet is deviated from a conveyance reference, may be caused. Also, damages, such as flaws, folds, and stains, may be caused to sheets.
Hitherto, a sheet feeding apparatus has been devised, which includes a pressure sensor adapted to monitor an internal pressure of a suction duct to detect completion of the suction of the sheet, as discussed in Japanese Patent No. 2750486.
Although the conventional sheet feeding apparatus employing such a pressure sensor and an image forming apparatus employing this conventional sheet feeding apparatus can detect the completion of suction of a sheet using the pressure sensor, the cost of the conventional sheet feeding apparatus and the image forming apparatus is increased due to costliness of the pressure sensor. Also, the pressure sensor is susceptible to variation in environmental conditions, such as temperature and humidity. Therefore, the conventional sheet feeding apparatus and the image forming apparatus employing the conventional sheet feeding apparatus have drawbacks in that when the environmental conditions drastically change, the completion of suction of a sheet cannot accurately be detected, and that a sheet cannot surely be separated and conveyed.
Also, in a case where the conventional pressure sensor is used, an actual measurement value is needed as a criterion for determining that the suction of a sheet to the suction conveyance belt is completed. For example, when an absolute value of an internal negative pressure of the suction duct becomes equal to or higher than 300 Pa, it is determined that the suction of a sheet to the suction conveyance belt is completed. Thus, an operation for controlling the apparatus is complicated. For example, accurate calibration of the pressure sensor is required for correcting variation in a characteristic thereof.